This invention relates to a projection optical system and a projection exposure apparatus having the same, for the manufacture of devices such as semiconductor devices, CCDs, or liquid crystal devices, for example. In another aspect, the invention is concerned with a device manufacturing method using such a projection exposure apparatus. The present invention is particularly suitably usable in a projection exposure apparatus of a step-and-repeat type or step-and-scan type.
The density of semiconductor devices such as a DRAM or CPU, for example, has increased considerably. In the latest devices, a circuit pattern of a size not greater than 0.25 micron is required. Projection exposure apparatuses, called a stepper, are widely used because of their ability of forming such a fine pattern precisely. In such steppers, a pattern of a reticle is illuminated with light of a short wavelength in the ultraviolet region, and it is projected through a projection optical system onto a semiconductor (silicon) wafer in a reduced scale, whereby a fine circuit pattern is formed on the wafer.
For precision transfer of a reticle pattern, many strict conditions are applied to the projection optical system. Since the pattern size being resolvable with the projection optical system is in an inverse proportion to the numerical aperture (NA), the designing should be made to enable enlargement of the numerical aperture. Additionally, the aberration must be corrected precisely over the whole region corresponding to the semiconductor chip.
The designing can be done with the aid of high-speed computers and designing software. Naturally, for production of a projection optical system, it is necessary to make every lens of the projection optical system very precisely, exactly in accordance with the design. But, in addition to this, much attention has to be paid to the glass material or materials to be used. Since the refractive index of a glass material has a large influence to the imaging characteristic of a projection optical system, the uniformity thereof is very strictly controlled, generally to an order of 10xe2x88x926 or less. Further, the birefringence or double refraction property of a glass material is largely influential to the imaging characteristic and, therefore, the magnitude thereof should be suppressed to about 2 nm/cm, as is known in the art.
However, with a glass material for a projection optical system which may have a largest diameter of 200 mm, it is very difficult to control the double refraction property so precisely, uniformly over the whole surface. Usually, for reasons to be described below, birefringence would be produced to some degree.
A first reason is attributable to the manufacturing process of a glass material. For light in the ultraviolet region, currently, a quartz (silica) glass is widely used. Thus, the following explanation will be made with reference to quartz glass. As compared with optical crystals, quartz glasses to be used as a lens glass material has no directionality in its structure. Therefore, in an idealistic state, no birefringence is produced.
However, in quartz glasses, birefringence which might be considered as being attributable to remaining stresses such as thermal hysteresis or impurities may be observed experimentally. While the manufacture of quartz glass may be based on a direct method, a VAD (vapor axial deposition) method, a sol-gel method, or a plasma burner method, for example, in any of these methods, it is difficult for current technology to reduce a mixture of impurities to a level that can be disregarded.
Further, in cooling a quartz glass being formed in a high temperature state, it may be possible to reduce the stress, resulting from differences in the way of being cooled between the surface portion and the inside portion of the glass material (i.e., the stress due to thermal hysteresis), to some extent by a thermal treatment such as annealing, for example. But, in principle, it is difficult to completely remove it.
Referring now to FIG. 24, the process of manufacturing a lens element to be used in a lithographic projection optical system will be described. First, an ingot 100 of quartz glass is produced with a revolutionally symmetric shape. It is then sliced with a required thickness, by which a disk-like member 101 is provided. Since the ingot 100 is produced constantly symmetrically with respect to its central axis 100a, distribution of impurities remaining in the member 101 or distribution of stresses therein due to thermal hysteresis appears, as a matter of course, symmetrically with respect to the central axis 101a. At a final stage, cutting and polishing are made to the member 101, whereby a lens element 102 is provided.
Now, distortion which may appear when impurities are mixed into the ingot 100 will be explained. FIG. 25 is a sectional view of the ingot 100. The peripheral hatching at 103 in this example shows a portion with a high impurity density. During an annealing process, the ingot 100 is heated. In the state with heat applied, the inside stress reduces to substantially zero. Through gradual cooling from that state, idealistically, a material without inside stress at room temperature can be provided. However, if impurities are mixed, the thermal expansion coefficient of the material changes. If the thermal expansion coefficient increases with the mixture of impurities, as a matter of course, it causes an increase of contraction during the cooling process.
As a result, although there is no stress in the heated state, the peripheral portion contracts largely with a temperature decrease. If particular attention is paid to the central portion of the glass material where a light flux is going to pass, it receives contraction from the peripheral portion as depicted by arrows in FIG. 25. That is, inside stresses are produced. The inside stress is a cause for birefringence.
A second reason is attributable to a change, with time, of quartz glass when used in a stepper. As is known in the art, if light from a short-wavelength light source such as a KrF or ArF laser is projected to a quartz glass, a phenomenon called xe2x80x9ccompactionxe2x80x9d may occur. Although details of how it occurs are not described here, what can be observed in that phenomenon is that the refractive index of the portion through which the light has passed increases but the volume of that portion decreases.
In FIG. 26, if laser light is projected to a hatched region 111 of the disk-like member 110, the volume of that portion is likely to decrease. Since the peripheral portion not irradiated with laser light is not influenced by compaction, as the whole, the central portion is likely to contract whereas the peripheral portion is likely to act against the contraction.
In a balanced state, therefore, when particular attention is paid to the central portion of the glass material where light passes, it receives tension forces from the peripheral portion as depicted by arrows in FIG. 27. Thus, inside stresses are produced. The inside stress is a cause for birefringence. The phenomenon described above may occur similarly in a projection optical system of a stepper. Since the phenomenon of compaction is particularly notable with use of ArF laser light, it may cause a large problem when a projection exposure apparatus with a light source of an ArF laser is practically developed.
As described, practically, it is very difficult to completely remove birefringence to be produced in a glass material. To the contrary, the requirement for birefringence in a stepper projection optical system is becoming strict, more and more. For providing a higher performance projection optical system, the number of lens elements constituting the projection optical system is increasing and, thus, the total glass material thickness is increasing. Therefore, even if the birefringence per unit length is kept to the above-described quantity (about 2 nm/cm), the total birefringence quantity of the system becomes large. Further, recent shortening in the wavelength of an exposure light source functions to enlarge the influence of birefringence.
Specifically, a comparison will be made to a case with the use of i-line light (wavelength 365 nm) and a case with the use of an ArF laser light source (wavelength 193 nm). If, for example the whole optical system has a birefringence property of 100 nm, in the case of i-line of 365 nm wavelength, it corresponds to a wavefront aberration of {fraction (100/365)}=0.27 wavelength. For an ArF laser light source of 193 nm wavelength, it corresponds to a wavefront aberration of {fraction (100/193)}=0.52 wavelength. Thus, for the same birefringence, the influence to an imaging characteristic is larger with a shorter wavelength.
As regards an optical glass material having birefringence in central symmetry, Japanese Laid-Open Patent Application, Laid-Open No. 107060/1996 shows the use of lens elements made of different glass materials having different birefringence quantities, and suggests reduction of adverse influence to the imaging characteristic by optimizing a combination of the glass materials. However, increasing a requirement to further improve the precision of a projection optical system cannot be met even by such a method. It is, therefore, desirable to cancel the birefringence itself of a glass material.
It is an object of the present invention to provide an improved projection optical system and/or an improved projection exposure apparatus having the same, for the manufacture of devices such as semiconductor devices, CCDs, or liquid crystal devices, for example, by which at least one of the problems described above can be solved.
It is another object of the present invention to provide a device manufacturing method using such a projection exposure apparatus.
In accordance with an aspect of the present invention, there is provided a projection optical system for projecting a pattern of a first object onto a second object, wherein said projection optical system is provided with birefringence correcting means for correcting birefringence of an optical element of said projection optical system.
Said birefringence correcting means may comprise at least one optical member having a predetermined form birefringence.
Said at least one optical member may be arranged so that a distribution, including a distribution of form birefringence produced by said at least one optical member, is effective to cancel the birefringence to be produced by an optical element of said projection optical system.
Said at least one optical member may be arranged to produce form birefringence on the basis of a diffraction grating having a period smaller than a wavelength used.
Said diffraction grating may be provided on the surface of the optical element of said projection optical system.
Said birefringence correcting means may comprise at least one optical member having a predetermined stress distribution.
Said at least one optical member may be arranged so that a distribution, including a distribution of stresses produced by said at least one optical member, is effective to cancel the birefringence to be produced by an optical element of said projection optical system.
In accordance with another aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination system for illuminating a first object with light; and a projection optical system as recited above, for projecting a pattern of the first object illuminated with the light from said illumination system, onto a second object for exposure of the same.
In accordance with a further aspect of the present invention, there is provided a projection exposure apparatus, comprising: illuminating means for illuminating a first object with slit-like light; scanning means; and a projection optical system as recited above, for projecting a pattern of the first object onto a second object while the first and second objects are simultaneously scanned in a widthwise direction of the slit-like light, at a speed ratio corresponding to a projection magnification of said projection optical system.
In accordance with a yet further aspect of the present invention, there is provided a device manufacturing method including a process for printing a device pattern on a substrate by use of a projection exposure apparatus as recited above.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.